1. Field of the Invention
This invention relates to a light-weight mirror blank for astronomical purposes, having a mirror plate of transparent vitreous silica or high-silica glass, a back plate of transparent vitreous silica, opaque vitreous silica or high silica glass and a supporting framework disposed between the mirror plate and the back plate and made of transparent vitreous silica, opaque vitreous silica or high-silica glass and consisting of a plurality of rows of tubes, the rows of tubes being in a staggered relationship to one another and the individual tubes being permanently joined to the mirror plate and back plate, the tube axes being parallel to one another and to the mirror axis. This invention further relates to a method of making a supporting framework for mirrors of the kind specified above.
2. Discussion Of Prior Art
U.S. Pat. No. 1,888,341 discloses a light-weight mirror blank for astronomical purposes, which has a supporting framework comprising a plurality of rows of tubes, the rows of tubes being in a staggered relationship to one another (FIG. 5). The tubes consist of transparent vitreous silica. The tube axes are parallel to one another and perpendicular to the mirror axis. The tubes are joined to one another at their junctions.
A light-weight mirror blank for astronomical purposes is known from U.S. Pat. No. 2,988,959, which has a mirror plate and a back plate between which a supporting framework of a plurality of rows of tubes is disposed, the tube rows being in a staggered relationship to one another. The tube axes are parallel to one another and parallel to the mirror axis. Glass is used as the material for this light-weight mirror blank. The individual tubes are permanently joined to the mirror plate and the back plate by means of a cement. The tubes can have a circular or rectangular or triangular cross section.
Lastly, another light-weight mirror blank for astronomical purposes is known from British Pat. No. 968,025, which has a mirror plate and a back plate as well as a supporting framework permanently joined to the two plates and made of tubes of hexagonal cross section assembled in honeycomb form. Transparent vitreous silica, opaque vitreous silica or a glass of high silica content is used as the material for the light-weight mirror blanks described therein.
In the known light-weight mirror blanks, either the supporting framework weight is very low while having at the same time a low transverse stability, or, if the transverse stability is high, the weight reduction of the supporting framework is only imperfect.